(1) Field of the Invention
This invention relates to a communication system and, more particularly, to a communication system for controlling optical communication.
(2) Description of the Related Art
Optical communication network technologies are nuclei for building a data communication network infrastructure. It is hoped that more advanced services will be provided in a wider area. Such technologies therefore are being developed at a rapid pace toward an information-oriented society.
In recent years the wavelength division multiplex (WDM) technique is widely used in optical communication. WDM is a system in which light with different wavelengths is multiplexed and in which a plurality of signals are transmitted simultaneously through one optical fiber.
In a WDM system, not only a main signal but also an optical supervisory signal at a frequency between about 1 and 150 MHz called an optical supervisory channel (OSC) is used. This OSC signal and a main signal are wavelength-multiplexed and transmitted. This OSC signal will be used to control setting and state supervision for lines, linear repeaters, and the like.
For example, this OSC signal not only controls setting and state supervision for an optical amplifier in a repeater but also detects a failure on a transmission line. In a WDM system, therefore, usually only a main signal is amplified by an optical amplifier (erbium-doped optical fiber amplifier (EDFA)) and is transmitted. An OSC signal transmitted does not pass through an optical amplifier.
An OSC signal is used as a control signal, so its sending level is relatively low to prevent it from interfering with a main signal. Furthermore, the transmission rate of an OSC signal is relatively low, so it does not produce much noise. This enables to set its receiving level to a small value near the noise limit of a receiver. In practice it is necessary to use a photocoupler to multiplex an OSC signal and a main signal or to separate them. The insertion loss of this photocoupler therefore must be taken into consideration. As a result, the transmission distance will be about 120 km.
Meanwhile, with recent optical communication systems, attention has been focused on an optical fiber amplifier (Raman amplifier) which uses a non-linear optical phenomenon in an optical fiber called Raman amplification. This amplifier uses the physical phenomenon of light with a wavelength different from that of incident light being scattered by vibration in a substance. With an optical fiber amplifier, optical amplification can be performed by sending strong pumping light into an entire transmission line (for example, in order to amplify an optical signal with a wavelength of 1.55 μm, pumping light with a wavelength of 1.45 μm should be sent).
By using such a Raman amplifier in a WDM system and performing optical amplification on both of main and OSC signals, an optical fiber with a length longer than before can be laid. This will enable to widen repeater spacing (the conventional transmission distance of about 120 km can be increased to 200 km or more).
Repeater spacing is wider than before. Therefore, when such a Raman amplifier in a WDM system is not operating, an OSC signal will attenuate in a transmission line and fall below the minimum receiving level.
An OSC signal is a control signal for setting etc. Therefore, only an OSC signal must first be sent to each node at the time of starting a system. However, a Raman amplifier is not operating at the time of starting a system, so an OSC signal cannot be transmitted.
Conventionally, the driving of Raman amplifiers at nodes is controlled individually, and the starting of the entire system is controlled after all the Raman amplifiers are driven. This will cause inefficiency in operation and maintenance.